1. Starlight: how the physics of electromagnetism provides a key to the stars

2. Light is the only information we get from most astronomical objects
Light is the only information we get from most astronomical objects. To understand these objects, we need to understand the physics of light and how it is produced.

3. First result: light is a wave (electromagnetic wave)
Wave characterized by wavelength, amplitude
DEMO

4. Amazing fact of nature: wide range of wavelengths of electromagnetic waves
EM radiation includes gamma rays, x-rays, ultraviolet,
Light, infrared, microwave, radio

5. Concept from physics crucial for astronomy: the spectrum of light
Will do in lab this week (or soon); DEMO

6. Spectra (plural of spectrum)
The solar spectrum
A fundamental measurement to extract more information from starlight
Spread out light according to wavelength
See Section 16.5

7. The Solar Spectrum as an astronomer would study it

8. What do the spectra of the Sun and stars tell us about those objects?
See Figure from book

9. The Physics of Spectrum Formation, Kirchoff’s Laws and Wien’s Law
Hot opaque solid or liquid produces a continuous spectrum
Hot, tenuous gas observed against dark background produces emission line spectrum
Cold, tenuous gas observed against bright background produces absorption spectrum
See Figure 16.6

10. Kirchoff’s Laws of Radiation

11. Kirchoff’s First Law + Wien’s Law
Hot, opaque objects produce continuous spectrum
The hotter the object, the bluer it is
Wien’s Law wmax = 2.9E-03/T
The hotter an object, the brighter it is
demo

12. Why does Wien’s Law look like that?
A physicist is bothered when he or she sees an equation like:
The form which emerges from fundamental equations of physics is:

13. Kirchoff’s First Law + Wien’s Law

14. Kirchoff’s 2nd Law: Emission Line Spectra
Wavelengths of emission lines unique “fingerprint” of element

15. Kirchoff’s Third Law: Absorption Spectra
See Figure 16.6

16. Starlight…application of spectroscopy to stars
Continuous spectrum gives surface temperature (Wien’s Law)
Spectral lines give chemical composition, temperature (also), speed of rotation (How?) and other properties
Examples of stellar spectra…what can we say?

17. Spectral classes of stars: O,B,A,F,G,K,M
What can you say about the temperatures of these stars?

18. Examples of stellar spectra
Question: apply Wien’s
Law to the O5 star. What
Can you say about its
Temperature (relative to the
Sun?)

19. With information provided by spectroscopy, we can search for correlations between stellar properties

20. What the data show: the Hertzsprung-Russell Diagram
Highest quality data from the Hipparchus spacecraft